1. Field of the Invention
This invention relates to homogeneous catalytic photochemical oxidation of alkanes by polyoxometalate catalysts.
2. Discussion of the Background Art
Catalysts are substances which facilitate reactions. At a given temperature, a catalyst accelerates the rate of a reaction. The term "catalyst" in this document is used in accordance with this standard meaning.
Catalysis is broken down into two different and unrelated classes of catalysis: (1) heterogeneous catalysis; and (2) homogeneous catalysis. In heterogeneous catalysis, the reaction mixture contains materials in at least two different physical states. Generally, the catalyst is in the form of a solid, and the substrate and product are in the form of either liquids and/or gases. Heterogeneous catalysis is characterized by high temperatures, high pressures and lower selectivity. This is the form of catalysis typically used in large-scale industrial applications (e.g., oil refining and coal liquefaction).
In homogeneous catalysis the reaction mixture is essentially made up of one physical phase. The catalyst, the substrate and the product are all typically dissolved in the reaction mixture. Homogeneous catalysis is characterized by lower temperatures, lower pressures and higher selectivity as compared to heterogeneous catalysis. It thus requires less energy and can be used on more sensitive substrates which could not withstand the temperature and pressure regimen of heterogeneous catalysis.
Because of the very nature of heterogeneous catalysis and heterogeneous catalysis, catalysts used in heterogeneous catalysis and catalysts used in homogeneous catalysis are considered to be two distinct and different systems in the art of catalysis. A heterogeneous catalyst is essentially always an insoluble solid material designed to withstand the adverse environment of heterogeneous catalysis without loss of its catalytic activity. A heterogeneous catalyst is designed to remain insoluble in the reaction medium to facilitate separation of the catalyst from the rest of the reaction mixture upon completion of the reaction.
Heterogeneous catalysts are typically inorganic materials selected on the basis of their ability to remain insoluble in the reaction medium, and withstand high temperatures and high pressures without change. Examples of typical heterogeneous catalysts include zeolites and transition metals supported on an inorganic matrix.
By contrast, homogeneous catalysts are materials which are selected because of their ability to dissolve in the reaction medium where they exhibit catalytic activity. Homogeneous catalysts are typically materials which, at a molecular level, possess a large organic component. Transition metals coordinated to various organic ligands are typical homogeneous catalysts. Homogeneous catalysts are thus selected on the basis of their ability to dissolve in the reaction medium and to exhibit selective catalytic activity.
Since only a limited number of elements are available, one will, of course, be able to find structurally similar materials used both in heterogeneous and homogeneous catalysts. But this overlap is only accidental, and, in the art of catalysis, a material's catalytic activity in a heterogeneous catalytic environment does not suggest its use in a homogeneous catalytic system, and vice versa.
The partial oxidation of organic substrates using either homogeneous or heterogeneous catalysts is one of the most important processes used to transform organic substrates into desired materials or intermediates used in the production of desired materials. For example, the partial oxidation reaction of hydrocarbons is one of the most applied processes for converting hydrocarbons into valuable chemical intermediates.
Alkanes, the most abundant class of organic materials, could be advantageously used in synthetic organic chemistry if they could be transformed into more reactive and useful molecules. In the past five years, interest in functionalization of saturated hydrocarbons, i.e. alkanes, has become intense, and the number of new processes for functionalizing alkanes has grown enormously. In particular, homogeneous catalytic systems for alkane functionalization have been sought because of their lowered energy consumption and simplicity. Thorough investigations of several homogeneous liquid-phase systems for alkane activation or functionalization at room temperature have been reported. These include organometallic systems that effect stoichiometric alkane C-H bond activation with unusual C-H cleavage selectivies, and metalloporphyrin systems that effect catalytic alkane functionalization with conventional C-H cleavage selectivities. See J. A. Smegal and C. L. Hill, J. Am. Chem. Soc., 1983, 105, 3515 and J. T. Groves and T. E. Nemo, J. Am. Chem. Soc., 1983, 105, 6243. There are now also many examples of organometallic alkane activation systems. The stability of these systems limits their use to only a few turnovers. It is to be noted that these catalysts always include organic groups ligated to a metal ion. The catalysts involved in the present invention, unlike all organometallic and metalloporphyrin species in the literature, contain no oxidizable organic structure in the polyoxometalate portion of the compound. This feature removes the element of oxidative instability which can be the success limiting factor for catalysts. The completely inorganic polyoxometalates of this invention combine the stability advantages of metal oxide and other heterogeneous catalysts with the selectivity and experimental tractability advantages of homogeneous catalysts.
As a separate development, a number of investigators have been investigating the ability of polyoxometalates to oxidize organic molecules. To date, however, these inorganic catalysts have not been used to oxidize or functionalize alkanes.
The polyoxometalates or polyoxoanions have been known for many years, but only recently has interest in these materials increased. To some degree, this is due to the fact that these materials have become more chemically well defined. In addition, since 1977, the polyoxometalates and particularly the heteropoly acids have received increasing attention as reagents or catalysts for redox processes involving organic substrates. The majority of these processes involve the use of the heteropoly acids as heterogeneous catalysts.
In terms of homogeneous processes, the polyoxometalates have generally functioned as catalysts for Pd reoxidation in organic oxidations related to the Wacker reaction. It is now recognized that photoassisted catalysis involving the polyoxometalates with visible or near-ultraviolet light facilitates a number of oxidations that are thermodynamically or kinetically unfavorable in the dark. Recently, the present inventor reported on polyoxometalate-catalyzed photooxidation and photodehydrogenation of organic compounds, in J. Am. Chem. Soc., 1985, 107, 5148-5157. However, in this paper, no alkanes were investigated as substrates.
Geletii and Shilov, in Kinet. Katal. 24, 486-489, reported on oxidation of methane in the presence of platinum salts and heteropoly acids. However, Geletii and Shilov clearly indicated that the oxidation of methane was catalyzed by platinum IV rather than by the heteropoly acid. No other alkanes were investigated.
Yamase, in Yuki Gosei Kagaku Kyokaishi 43, 249-261, reported on photochemistry of polyoxometalates as homogeneous photocatalysis for redox reactions of various organic compounds. Yamase discloses alkene substrates, but does not disclose the use of alkanes as substrates in his reactions. The alkenes give rise to photodimeric products in relatively high yields, whereas the alkanes involved in the present invention give little if any dimeric coupling products.
The following references are also related to the present invention, but in each case the disclosure therein is different from the present invention in that alkanes are not used as substrates, and further, in most cases, heterogeneous rather than homogeneous catalysis is involved:
1. J. Chem. Soc. Dalton Trans. 1985, 395-399 "HeteropolyTungstates as Catalysts for the Photochemical Reduction of Oxygen and Water";
2. J. Phys. Chem. 1984, 88, 4210-4213, "Photocatalytic Alcohol Dehydrogenation Using Ammonium Heptamolybdate";
3. J. Chem. Soc. Dalton Trans. 1984, 793-799 "Solution Photochemistry of Tetrakis(tetrabutylammonium) Decatungstate(VI) and Catalytic Hydrogen Evolution from Alcohols";
4. Inorganica Chimica Acta, 87 (1984) 177-180, "Photochemistry of Heteropoly Electrolytes: the 1:12 Tungstates";
5. J. Chem. Soc., Chem. Commun., 1982, 12-13 "Photocatalytic Oxidation of Organic Compounds using Heteropoly Electrolytes of Molybdenum and Tungsten";
6. Inorganica Chimica Acta, 46 (1980) 155-158, "Photochemistry of Heteropoly Electrolytes. The 18-Molybdodiphosphate";
7. J. Chem. Soc. Dalton Trans. 1986, 1669-1675 "Photoredox Chemistry of Keggin Dodecatungstoborate [BW.sub.12 O.sub.40 ].sup.5- and Role of Heterogeneous Catalysis in Hydrogen Formation";
8. Inorg. Chem. 1986, 25, 4386-4389, "Vanadium-Sensitized Photochemistry of Heteropoly Compounds. Mixed Molybdo- and Tungstovanadates";
9. Inorg. Chem. 1985, 24, 439-441, "Photocatalytic Generation of Hydrogen by 1:12 Heteropolytungstates with Concomitant Oxidation of Organic Compounds";
10. Russian Chemical Reviews, 51 (11), 1982, 1705-1088 "Heteropolyacids in Catalysis". This reference pertains to thermal, catalytic, primarily homogeneous, processes.
Currie et al, U.S. Pat. No. 4,612,301 is of interest in connection with the present invention in that it discloses heteropoly acids as catalysts for alcohol conversion. Alkanes are not disclosed as substrates therein.
Bergman et al, U.S. Pat. No. 4,511,745, relates to a process for functionalizing alkanes using an organometallic compound. The organometallic compound is quite different from the catalyst used in the present invention.
In spite of the above publications and patents, there has continued to exist a need for new and improved methods for functionalizing alkanes, particularly methods which give rise to new selectivities, improved yields, or new types of products based on alkanes, as compared to the prior art methods.